1. Technical Field
The present invention relates to the preparation of diffuse, optically black, plasma sprayed coatings containing a carbonyl metal. The coatings are stable in a space environment and have a high solar absorptance and high infrared emittance.
2. Description of the Prior Art
There is a critical need, in space systems, for coating materials that are stable in the harsh environment of space; that is, that are resistant to a hard vacuum, high energy UV radiation, thermal cycling, electron bombardment, and atomic oxygen. These coatings must survive for the life of the space mission, which can be as long as fifteen years. Current coating materials are either organic based or use a ceramic glaze base. Organic materials degrade rapidly in a space environment, while ceramic glaze materials usually need a high temperature heat treatment. To save weight, the substrate in a space system is usually very thin. Such heat treatment can result in distortion of thin metal structures.
U.S. Pat. No. 3,231,416 discloses the preparation of zirconia/boron ablation coatings to provide protection to the outer surface, and especially to the thin walled nose section, of high velocity missiles or space vehicles. The coatings are obtained by plasma spraying onto the surface a powder mixture comprising 50 to 70 volume percent zirconia and 30 to 50 volume percent boron. The zirconia has a particle size of about 325 mesh and the boron has a particle size of about 100 mesh. There is no suggestion in the patent that the coatings have optically black properties.
U.S. Pat. No. 3,753,745 discloses coatings for high temperature environments, such as a furnace, prepared by flame or plasma spraying a powder mixture of zirconium oxide and 20-60 weight percent nickel and/or nickel oxide onto a substrate. The nickel or nickel oxide in the coating densifies the coating reducing porosity to less than 5%.
U.S. Pat. No. 3,753,666 discloses plasma spraying iron titanate powder (Fe.sub.2 O.sub.3.TiO.sub.2) onto a substrate of a platinum-group metal to provide a high emittance coating. The purpose is to control the surface emittance of radioisotope containment vessels for aerospace applications. The survivability of the vessel depends PG,4 upon the temperature of the surface of the vessel. This temperature can be lowered to an acceptable level by the use of a high emittance coating.
U.S. Pat. No. 4,335,190 discloses applying a bond coat to a substrate by plasma spraying. The bond coat is NiCrAlY or CoCrAlY and provides a base for a ceramic top coat such as calcia stabilized zirconia or magnesia stabilized zirconia. The ceramic top coat is also plasma sprayed.
U.S. Pat. No. 3,989,872 discloses a plasma spray powder which comprises a yttria stabilized zirconia core encased in a thin calcia shell. The calcia shell promotes interparticle bonding of the zirconia.
U.S. Pat. No. 4,055,705 is similar to U.S. Pat. No. 4,335,190 and discloses a substrate, a bond coating consisting essentially of NiCrAlY covering said substrate, and a thermal barrier coating consisting essentially of zirconia stabilized with another oxide, such as calcium oxide, magnesium oxide, and yttrium oxide. Both the bond coat and the barrier coating are plasma sprayed.
U.S. Pat. No. 4,269,903 also discloses the combination of a substrate, a NiCrAlY bond coat on said substrate, and a barrier layer of yttria stabilized zirconium oxide on said bond coat. The article of this patent also contains a top abradable layer consisting essentially of porous stabilized zirconia resulting from the thermal decomposition of an organic filler material co-deposited with stabilized zirconia on the barrier layer.
U.S. Pat. No. 4,645,716 discloses applying a bond coat by flame spraying an alloy of nickel or cobalt containing chromium and/or aluminum. A ceramic composition is flame sprayed onto the bond coat, the ceramic composition comprising zirconium oxide, yttrium oxide, and titanium oxide.